1. Field of the Invention
The present invention relates generally to the field of ascorbic acid production. More particularly, it relates to a process for the production of L-ascorbic acid from yeast, including recombinant yeast.
2. Description of Related Art
L-ascorbic acid (Vitamin C) is a powerful water-soluble antioxidant that is vital for growth and maintenance of all tissue types in humans. One important role of ascorbic acid is its involvement in the production of collagen, an essential cellular component for connective tissues, muscles, tendons, bones, teeth and skin. Collagen is also required for the repair of blood vessels, bruises, and broken bones. Ascorbic acid helps regulate blood pressure, contributes to reduced cholesterol levels, and aids in the removal of cholesterol deposits from arterial walls. Ascorbic acid also aids in the metabolization of folic acid, regulates the uptake of iron, and is required for the conversion of the amino acids L-tyrosine and L-phenylalanine into noradrenaline. The conversion of tryptophan into seratonin, the neurohormone responsible for sleep, pain control, and well-being, also requires adequate supplies of ascorbic acid.
A deficiency of L-ascorbic acid can impair the production of collagen and lead to joint pain, anemia, nervousness and retarded growth. Other effects are reduced immune response and increased susceptibility to infections. The most extreme form of ascorbic acid deficiency is scurvy, a condition evidenced by swelling of the joints, bleeding gums, and the hemorrhaging of capillaries below the surface of the skin. If left untreated, scurvy is fatal.
Although intestines easily absorb ascorbic acid, it is excreted to the urine within two to four hours of ingestion. Therefore, it cannot be stored in the body. L-ascorbic acid is produced in all higher plants and in the liver or kidney of most higher animals, but not humans, bats, some birds and a variety of fishes. Therefore, humans must have access to sufficient amounts of ascorbic acid from adequate dietary sources or supplements in order to maintain optimal health.
Food sources of ascorbic acid include citrus fruits, potatoes, peppers, green leafy vegetables, tomatoes, and berries. Ascorbic acid is also commercially available as a supplement in forms such as pills, tablets, powders, wafers, and syrups.
L-Ascorbic acid is approved for use as a dietary supplement and chemical preservative by the U.S. Food and Drug Administration and is on the FDA's list of substances generally recognized as safe. L-Ascorbic acid may be used in soft drinks as an antioxidant for flavor ingredients, in meat and meat-containing products, for curing and pickling, in flour to improve baking quality, in beer as a stabilizer, in fats and oils as an antioxidant, and in a wide variety of foods for ascorbic acid enrichment. L-Ascorbic acid may also find use in stain removers, hair-care products, plastics manufacture, photography, and water treatment.
The enzymes of the biosynthetic pathways leading to ascorbic acid have not been identified yet to completion. Current understanding of the physiological pathways in plants and animals is shown in FIG. 1.
In animals, D-glucose serves as the first precursor and the last step is catalyzed by a microsomal L-gulono-1,4-lactone oxidase. The enzyme has been isolated and characterized from different sources. The gene from rat has been cloned and sequenced (Koshizaka T. et al., 1998, J. Biol. Chem. 263, 1619-1621.)
Two discrete pathways have been reported for ascorbic acid synthesis in plants. In one pathway, L-ascorbic acid is synthesized from D-glucose via L-sorbosone (Loewus M. W. et al., 1990, Plant. Physiol. 94, 1492-1495). Current evidence suggests that the main physiological pathway proceeds from D-glucose via L-galactose and L-galactono-1,4-lactone to L-ascorbic acid (Wheeler G. L. et al. 1998, Nature, 393, 365-369). The last two steps are catalyzed by the enzymes L-galactose dehydrogenase and L-galactono-1,4-lactone dehydrogenase. Also in this case, the last enzyme has been isolated and characterized, and the gene from Brassica oleracea has been cloned and sequenced (Østergaard J. et al. 1997, J. Biol. Chem., 272, 30009-30016).
For use as a dietary supplement, ascorbic acid can be isolated from natural sources or synthesized chemically by the oxidation of L-sorbose as in variations of the Reichstein process (U.S. Pat. No. 2,265,121).
It remains desirable to have methods for the production of ascorbic acid by convenient processes. Two main requirements in the production of ascorbic acid are that the synthesis should be enantioselective, because only the L-enantiomer of ascorbic acid is biologically active, and that the environment of the final steps of the process should be non-oxidative, because ascorbic acid is very easily oxidized.
One possible approach is the production of L-ascorbic acid from microorganisms. Microorganisms can be easily grown on an industrial scale. Although the production of L-ascorbic acid from microorganisms and fungi has been reported in the past, recent evidence proves that L-ascorbic acid analogues, and not L-ascorbic acid, are found (Huh W. K. et al. 1998, Mol. Microbiol. 30, 4, 895-903)(Hancock R. D. et al., 2000, FEMS Microbiol. Let. 186, 245-250)(Dumbrava V. A. et al. 1987, BBA 926, 331-338)(Nick J. A. et al., 1986, Plant Science, 46, 181-187). In yeasts (Candida and Saccharomyces species), the production of erythroascorbic acid has been reported (Huh W. K. et al., 1994, Eur. J. Biochem, 225, 1073-1079)(Huh W. K. et al., 1998, Mol. Microbiol. 30, 4, 895-903). In such yeasts, a physiological pathway has been proposed proceeding from D-glucose via D-arabinose and D-arabinono-1,4-lactone to erythroascorbic acid (Kim S. T. et al., 1996, BBA, 1297, 1-8). The enzymes D-arabinose dehydrogenase and D-arabinono-1,4-lactone oxidase from Candida albicans as well as S. cerevisiae have been characterized. Interestingly, L-galactose and L-galactono-1,4-lactone are substrates for these activities in vitro.
In vivo production of L-ascorbic acid has been obtained by feeding L-galactono-1,4-lactone to wild-type Candida cells (International Patent Application WO85/01745). Recently it has been shown that wild-type S. cerevisiae cells accumulated intracellularly L-ascorbic acid when incubated with L-galactose, L-galactono-1,4-lactone, or L-gulono-1,4-lactone (Hancock et al., 2000, FEMS Microbiol. Lett. 186, 245-250)(Spickett C. M. et al., 2000, Free Rad. Biol. Med. 28, 183-192).
Wild-type Candida cells incubated with L-galactono-1,4-lactone accumulate L-ascorbic acid in the medium, suggesting that this yeast has a biological mechanism for the release of the intracellular accumulated L-ascorbic acid; indeed, L-ascorbic acid is a complex molecule and it is scientifically reasonable that its accumulation in the medium is not related to a simple diffusion process, but should depend on facilitated or active transport. This conclusion is supported by the identification and characterization of L-ascorbic acid transporters in higher eukaryotic (mammalian) cells (Daruwala R. et al., 1999, FEBS Letters. 460, 480-484). However, L-ascorbate transporters have not been described among the yeast genera. Nevertheless, while Candida cells growing in media containing L-galactono-1,4-lactone accumulate L-ascorbic acid in the medium, accumulation in the medium of L-ascorbic acid from wild-type S. cerevisiae cells has, surprisingly, never been described.
A desirable method for the large-scale production of ascorbic acid comprises the use of genetically engineered microorganisms (i.e., recombinant microorganisms). Both prokaryotic and eukaryotic microorganisms are today easily and successfully used for the production of heterologous proteins as well as for the production of heterologous metabolites. Among prokaryotes, Escherichia coli and Bacillus subtilis are often used. Among eukaryotes, the yeasts S. cerevisiae and Kluyveromyces lactis are often used. Despite the great success of these hosts, only one example has been described for the production of L-ascorbic acid by transformed microbial cells. Since only eukaryotic cells are natural L-ascorbic acid producers, it is even more surprising that only a prokaryotic transformed microbial host has been described to lead to the intracellular accumulation of L-ascorbic acid. Lee et al. (Appl. Environment. Microbiol., 1999, 65, 4685-4687), showed that the cloning of the S. cerevisiae gene encoding D-arabinono-1,4-lactone oxidase into E. coli allows the production of L-ascorbic acid from E. coli incubated with L-galactono-1,4-lactone. Accumulation of L-ascorbic acid was observed only at the intracellular level.
No experimental data have been described in the literature about the production of L-ascorbic acid from transformed eukaryotic microorganisms. Østergaard et al. cloned the gene encoding L-galactono-1,4-lactone dehydrogenase from cauliflower in the yeast S. cerevisiae (J. Biol. Chem., 1997, 272, 48, 30009-30016). While, in vitro, the authors found L-galactono-1,4-lactone dehydrogenase activity in the yeast cell extract (cytochrome c assay, see Østergaard et al.), no production of L-ascorbic acid was proven in vivo.
Berry et al., International Patent Appln. WO 99/64618 discuss the potential use of the plant biosynthetic pathway of ascorbic acid; special emphasis is given to the activity catalyzing the conversion of GDP-D-mannose to GDP-L-galactose. However, characterization of the enzyme catalyzing this step has not been presented in detail. An overexpressed E. coli homologue turned out to be inactive.
Smirnoff et al., WO 99/33995, discuss the use of L-galactose dehydrogenase for production of ascorbic acid. The enzyme was purified from pea seedlings and the N-terminal protein sequence was determined. The complete sequence is not known and has not yet been reported. The L-galactose dehydrogenase enzyme partial sequence was 72% identical to amino acids 5-22 of an unidentified putative coding sequence from Arabidopsis thaliana, accession no. 3549669.
Roland et al., U.S. Pat. Nos. 4,595,659 and 4,916,068, discuss the use of non-recombinant Candida strains to convert L-galactonic substrates to L-ascorbic acid. Roland et al. described the responsible enzyme as L-galactono-1,4-lactone oxidase.
Kumar, WO 00/34502, discusses the production of L-ascorbic acid in Candida blankii and Cryptococcus dimennae yeast capable of using 2-keto-L-gulonic acid as a sole carbon source in the production. Kumar specifically excludes the production from yeast by a pathway involving L-galactonolactone oxidase or by conversion of L-galactonic precursors.
It remains desirable to have methods for the production of ascorbic acid by a convenient fermentation process.